The present invention relates to a continuous press. More particularly this invention concerns belt-type press used for making wood panels, e.g. particleboard, flakeboard, plywood, chipboard, and the like.
A standard belt press has a frame having horizontally extending and vertically spaced upper and lower heated platens defining a horizontally extending gap having an upstream end and a downstream end, vertically spaced upper and lower upstream drums rotatable about respective horizontal axes at the upstream end, vertically spaced upper and lower downstream drums rotatable about respective horizontal axes at the downstream end, and upper and lower endless belts spanned over the respective upper and lower drums and each having a working stretch lying between the platens and a return stretch. Upper and lower sets of rollers engaged between the working stretches and the respective platens can be recirculated as the belts are advanced to move with the working stretches horizontally in a transport direction to displace a workpiece in the direction through the gap so that the working stretches are supported on the respective platens by these rollers.
As a rule, the belts are so wide that they extend transversely, that is perpendicular to their movement direction, past the rollers. Edge regions of the belts therefore are not in contact with the rollers and therefore the heat from the respective platens is not transmitted to them so that they are substantially cooler, often as much as 80xc2x0 C. cooler, than the central region.
As a result the stainless-steel belts are going to be, in effect, longer in a central region than in these cooler edge regions. The result is that the tension in the belts is greater in the effectively shorter edge regions than n the center, leading to premature failure of the belts at the overstressed edge regions.
The solution to this problem is to use narrower belts that do not extend past the ends of the support rollers. While this construction ensures that the entire belt is at roughly the same temperature so that the edge regions have the same effective length at the center region, it has the considerable disadvantage that it leaves the rollers exposed at their ends. Hence the rollers can get fouled so that the press must be cleaned more often.
It is therefore an object of the present invention to provide an improved belt-type particleboard press.
Another object is the provision of such an improved belt-type particleboard press which overcomes the above-given disadvantages, that is which ensures adequate protection of the belt-support rollers while at the same time preventing excessive localized stress in the belts.
A continuous belt press has a frame having horizontally extending and vertically spaced upper and lower platens defining a horizontally extending gap having an upstream end and a downstream end. Vertically spaced upper and lower upstream drums are rotatable on the frame about respective horizontal drum axes at the upstream end, and vertically spaced upper and lower downstream drums are rotatable on the frame about respective horizontal drum axes at the downstream end. Upper and lower endless steel belts spanned over the respective upper and lower drums each have a working stretch lying between the platens and a return stretch. Upper and lower sets of rollers are engaged between the working stretches and the respective platens. A drive connected to the drums advances the belts to move the working stretches horizontally in a transport direction to displace a workpiece in the direction through the gap. According to the invention each belt has a pair of edges projecting transversely past the respective rollers. Furthermore at least one of the drums of each of the belts has a substantially cylindrical central region of a predetermined central diameter bearing through the respective rollers on the working stretch of the respective belt and a pair of edge formations of smaller diameter than the central-region diameter lying outside the central region and in engagement with the edges of the respective belt.
Thus with this system the distance around the drums is in effect shorter at the belt edges so as to accommodate these edges which, as a result of being cooler, are shorter than the central regions of the belts. The belts will therefore be under substantially the same tension over their entire widths, not under greater tension at the edges as in the prior-art systems, so that they will have a much longer service life. Furthermore it is possible to use relatively wide belts, thereby effectively protecting the rollers and drums, something that is extremely important with, for instance, the highly sticky binder used in the production of medium-density-fiber panels.
The one drum of each of the belts in accordance with the invention is provided in its central region with friction pads forming a cylindrical drum outer surface and with end rings flanking the friction pads and forming the edge formations. These pads are normally of some high-friction material that is bonded to flexible metal mounting strips that are themselves bolted to the respective drums.
It is within the scope of this invention for both of the drums of each belt to be shaped according to the invention, but it is also possible for only one drum of each belt to be thus shaped.
The edge formations according to the invention can form radially outwardly convex curved surfaces engaging the respective edges. Alternately they form frustoconical surfaces engaging the respective edges and centered on the respective axes.